The present invention relates to optical projectors of the kind which supply a beam of radiation, the total angular field of which is pre-determined in at least one reference plane, said angular field containing all the radiant flux and incorporating a source of radiation associated with a reflecting optical collector system, a conoid or pyramidoid mirror which has at least the said reference plane as a plane of symmetry and the minor base of which, in conjunction with said reflecting optical collector system, entirely contains optically said source of radiation. Throughout the present specification the expression "conoid or pyramidoid mirror" is intended to mean a mirror the shape of which is that of the lateral surface of a geometric solid which terminates in two mutually parallel end-surfaces of different area, which are permeable to radiation, the cross-sectional area of the solid increasing progressively from its minor base to its major base, the two bases being connected together by reflective lateral faces and the geometric solid in question having at least two planes of symmetry perpendicular to the bases. The intersection between these two planes of symmetry forms the central optical axis of such conoid or pyramidoid mirror. Furthermore, the word "pyramidoid" means more particularly a mirror of this type in which the bases are substantially polygonal and the adjoining reflective lateral faces are plane or substantially plane (that is to say slightly curved or cambered).
It is already known to fix the dimensions of a conoid or pyramidoid mirror in such a way that all the flux received at the minor base (less reflection losses) is projected from the major base. The dimensional ratio proposed for use may be written as ##EQU2## WHERE D and d are the cross-sectional lengths of the major and minor bases of the mirror respectively when the cross-section is taken centrally, .gamma. is half the mean apex angle in this cross-sectional plane of the conoid or pyramidoid mirror, and .theta.' is half the total angular field of the projected flux, which is predetermined, the field in question containing the whole of the projected flux.
The dimensional relationships here proposed do not take into account the aperture angle of the radiant beam received at the minor base of the mirror, with the result that, when this angle is greater than 180.degree., the proposed relationships do not allow the best optimization to be achieved, that is to say the least ratio D/d and the minimum length of the conoid or pyramidoid mirror, which would result in smaller overall size and which would reduce the number of reflections in the mirror.
It is an object of the invention to remove or at least to minimize this drawback.